We're obsessed with it, and never seem to have enough. But scientists don't know how to explain it--or even if it really exists

Sometimes it flies, sometimes it crawls, but it always passes inexorably. We mark it, save it, waste it, bide it, race against it. We measure it incessantly, with a passion for precision that borders on the obsessive. Time is so vitally enmeshed with the fabric of our existence, in fact, that it's hard even to conceive of it as an independent entity--and when we try, the result is less than enlightening. Pondering the mystery of what time really is, St. Augustine wrote in his Confessions, "If no one asks me, I know; but if any person should require me to tell...